1. Technical Field
The present invention relates generally to radio frequency (RF) signal circuitry, and more particularly, to RF front end circuits with antenna diversity for multipath mitigation.
2. Related Art
Wireless communications systems find application in numerous contexts involving information transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. These systems include cellular telephones and two-way radios for distant voice communications, as well as shorter-range data networks for computer systems, among many others. In general, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. For wireless data networks, such standards include Wireless LAN (IEEE 802.11x), Bluetooth (IEEE 802.15.1), and ZigBee (IEEE 802.15.4).
In general, these communications modalities transmit and receive signals on a single channel or frequency. In order to share the single channel, the transmit and receive signals are time-domain duplexed. That is, for a predetermined period of time, one transmitter of a first communications node generates a burst signal, and for another predetermined period, the other transmitter at a counterpart communications node generates another burst signal to be detected by the receiver of the first communications node. It is understood that the transmit signals and the receive signals do not overlap in the time domain. Where the receiver detects errors in the burst signal via checksums and other well-known error detection/correction techniques, the other transmitter may be directed to retry. Errors may be caused in part by increased noise from the surrounding environment, obstacles and so forth. If there are a substantial number of retry attempts, data throughput is decreased, or the communications link may cease altogether.
Signal reception problems in WLAN and other systems are typically attributable to multi-path propagation phenomena, where a single signal reaches the antenna via two or more different paths because of obstacles between the transmitter and the counterpart receiver. At the RF signal level, destructive interference and phase shifts may occur. Specifically, the signals may be of differing phases and when combined at the receiver, may be weakened to the point of being unrecoverable by the receiver, thus forcing additional transmission retry attempts.
One approach to solve this problem employs two antennas for the same communications node that are physically separated from each other. The probability that the RF signals will reach both of the antennas with different amplitudes and phases and have a destructive effect on each other upon receipt is known to be miniscule, and the dual antenna configuration is understood to exploit this low probability. In most implementations, there are two receive chains, each connected to a separate antenna. The receiver may further process the stronger of the two detected signals. This configuration is known as receive antenna diversity. Alternatively, or in conjunction with receive antenna diversity, two spatially separated antennas may be connected to separate transmit chains of the same communications node in a configuration known as transmit antenna diversity, and is also known to mitigate multipath phenomena, even with a counterpart single antenna receiver.
Conventional implementations of receive antenna diversity utilize a dual pole, dual throw (DPDT) RF switch with a first pair of terminals connected to the two antennas and a second pair of terminals connected to the receive chain and the transmit chain. Thus, the switch connects only one of the antennas to the receive chain at a time, with the receiver selecting the antenna with the higher received power level. It is possible to implement transmit antenna diversity in this configuration as well, as the DPDT switch connects only one of the antennas to the transmit chain at a time. A special algorithm may be used to select the receive antenna with the higher power level that involves switching from one antenna to another between transmission bursts. However, during this initial estimation period, data throughput of the communications link may be lower in comparison to the steady state condition when the antenna with better reception has been determined. Besides utilizing DPDT switches, single pole, dual throw switches may be substituted, or any other suitable switch configurations.
Transceivers for WLAN, Bluetooth, Zigbee, and the like typically do not generate sufficient power or have sufficient sensitivity necessary for reliable communications. Additional signal conditioning is therefore necessary, and so the receive chain includes a low noise amplifier and the transmit chain includes a power amplifier. This circuitry between the antenna and the transceiver is also referred to as a front end module or circuit, and for those with antenna diversity features, the aforementioned DPDT switch is included therein along with the low noise amplifier and the power amplifier. The DPDT switch is on the antenna side, while the low noise amplifier and the power amplifier is on the transceiver side.
In the transmit mode, the power amplifier is turned on, while the low noise amplifier is turned off, with the transmit signal applied to the power amplifier. The DPDT switch is set so that the power amplifier is connected to the first of the pair of antennas or the second of the pair of antennas. The transmit signal amplified by the power amplifier is thus selectively applied to the first or the second one of the pair of antennas.
In the receive mode, the low noise amplifier is turned on, while the power amplifier is turned off. The DPDT switch is set so that the low noise amplifier is connected to the first of the pair of antennas or the second of the pair of antennas, such that the signal received on either one of the pair of antennas is amplified by the low noise amplifier for further processing by the RF transceiver and the baseband circuitry.
There are a number of deficiencies associated with such conventional front end modules with antenna diversity, however. One known problem is the insertion loss between either of the pair of antennas and the low noise amplifier attributable to the DPDT switch. Consequently, the noise figure of the receive chain is elevated. Furthermore, a modular fabrication technique is necessary to separate the power amplifier and the low noise amplifier from the DPDT switch. In particular, the power amplifier circuitry and the low noise amplifier circuitry is fabricated on one semiconductor die utilizing a Gallium Arsenide (GaAs) or Silicon Germanium (SiGe) substrate with hetero-junction bipolar transistors (HBT). The DPDT switch is fabricated on another semiconductor die utilizing a GaAs substrate with high electron mobility transistors (HEMT) or metal semiconductor field effect transistors (MESFET). It is possible for both HBT and HEMT transistors to be fabricated on a single die with a composite GaAs substrate, but at a greater cost. The DPDT switch therefore represents a significant constraint on the design and configuration of the front end circuit.
Accordingly, there is a need in the art for an improved RF front end circuit with antenna diversity for multipath mitigation.